1. Field of the Invention
The present invention relates generally to wave soldering apparatus and, more particularly, to a timer for measuring the solder contact time developed by the apparatus in soldering a device.
2. Description of Related Art
Where it is necessary to provide soldering of a large number of electrical, electronic components, subsystems or systems together such as on a printed circuit card, for example, it is impractical to attempt this by a point-to-point manual soldering technique. Instead, such large number of connections are soldered together by apparatus operating on what is referred to as a "wave" soldering principle. In a wave soldering machine, a printed circuit board to be soldered is moved by a conveyor through an enclosure on an inclined path past a fluxing station, a preheating station, and, finally, a station at which a wave of solder is caused to well upwardly and contact the various parts to be soldered. A very important wave soldering parameter for insuring efficient and reliable soldered connections is the dwell time of the devices to be soldered in the soldering wave.
In the past, the only technique known for measuring dwell time was by indirect means in which a wave "footprint" measurement device was used and a plot was developed from empirical data. Specifically, a glass plate having a set of grid lines of known dimensions is passed along the wave soldering machine conveyor and the contact of the solder wave is measured by the eye counting grid squares or lines of contact, for example. Then, it is a simple matter of calculating dwell time by multiplying the thickness of the wave as measured on the plate by 60 seconds per minute and dividing by the conveyor speed in inches per minute, for example. Although this technique is accurate, it leaves much to be desired in that for a number of different reasons the dwell time can change quickly during a run, for example, and also from day to day depending upon temperature variation in the solder, the change in composition of the solder, or other factors.
There is a further and undesirable effect termed "drag-out", that can result from the wetting ability of materials used and component lead configuration, in which the solder wave is pulled out of shape as the solder joints leave the wave. When this happens, the dwell time for certain joints will be significantly longer than for others. The glass plate is not wetted by the solder, and therefore, the known "footprint" technique does not indicate the presence of drag-out.
It is, therefore, desirable to be able to provide a solder dwell time measurement means which operates quickly, automatically, does not rely upon human observational techniques and calculations for achievement, and can exhibit the presence of drag-out.